QSys/NIOS triple RS232 port

First of all remind that RS232 refers to the physical layer, not the Qsys component which actually is a generic UART. If you really need RS232 you can't use directly the LVTTL signals on expansion connector, but you need a RS232 transceiver in order to translate voltage levels.

Besides this, you can add as many serial ports you want, provided your fpga has enough resources and your Nios can handle the increased code size and load.

Qsys will take care of address space, by placing each new component at a different base address and avoiding overlaps. So you won't have conflicts unless you manually change base addresses. Basic UART component has only a bunch of registers (IIRC address range is 8), so you definitely won't fill the memory address space.